As a playback control apparatus of this type, JP-A-6-309854 is already known for instance. In this technique, there is disclosed an audio system 101 shown in FIG. 6, and this audio system 101 is constituted by a playback device (CD player in an example of Patent Document 1) 102, an adapter 110, an on-vehicle antenna 104, an FM receiver (e.g., radio) 103, and left and right speakers 103a, 103b. This adapter 110 has a FM transmitter 107 inside. The FM transmitter 107 has a signal modulation circuit for FM-modulating a sound signal outputted from the CD player 102, and a carrier generator generating an FM carrier wave, and converts the sound signal outputted from the CD player 102 into a radio wave in an FM broadcast band of the radio 103 and outputs it. And, since this radio wave is received by the on-vehicle antenna 104, the sound signal of the CD player 102 is outputted from the left and right speakers 103a, 103b by tuning a frequency of this radio wave. By doing like this, it is possible to play back the CD player 102 by utilizing the radio 103 and the speakers 103a, 103b provided in an automobile.
Further, as shown in a block diagram of FIG. 7, this adapter 110 has a plug 112 capable of inserting into a cigarette lighter socket 127 installed in the automobile and, if an engine of the automobile is switched ON, an electric source is supplied from the cigarette lighter socket 127 to the adapter 110 through the plug 112. And, if the electric source is supplied to the adapter 110, it is converted into a desired voltage in an electric source circuit 111, and the electric source is supplied to the CD player 2 and the FM transmitter 107 through a connector 113.
Subsequently, a playback method of this CD player 102 will be described. A driver first connects the CD player 102 and the adapter 110 via a cable. Of course, if the CD player 102 and the adapter 110 are always connected, this connection work is unnecessary. Thereafter, the driver switches ON the engine of the automobile. Whereupon, since an electricity is applied to the cigar lighter socket 127, the electric source is supplied to the adapter 110 and the CD player 102. And, the driver presses down a playback button of the CD player 102. Whereupon, as mentioned above, the sound of the CD player 102 is outputted from the left and right speakers 103a, 103b.     Patent Document 1: JP-A-6-309854
However, in the above-mentioned technique, after applying the electricity to the sound playback device, since it is necessary to send playback instructions to the sound playback device by an operation of a user, the operation required before a sound output is troublesome because there is a problem that it is inconvenient. For example, in the case that the CD player is played back by the above audio system, the driver must press down the playback button of the CD player after switching ON the engine of the automobile. Therefore, a pressing-down operation of the playback button becomes a labor for the driver and, further if this pressing-down operation is performed during the movement of the automobile, it is not desirable from a viewpoint of safety.
Thus, there is a need in the art for a playback control apparatus that can contrive an improvement in operability or convenience of the user in a system in which the sound that is played back by a sound playback device is outputted from a speaker provided externally of the device.